1. Field of the Invention
This invention relates to an insulating material for a wiring substrate and a method of producing a multi-layered wiring substrate using the insulating material.
To reduce the size of an electronic device and to improve its operation speed, a higher density wiring substrate, to which a semiconductor integrated circuit is to be mounted, is necessary with the improvement in the integration density of the semiconductor integrated circuit.
The integration density of the wiring substrate has been improved from a single layer printed board at the outset to a multi-layered printed board and to a surface package, and a multi-chip module (MCM) capable of coping with a semiconductor chip package and capable of attaining a higher density has been developed.
Although a ceramic wiring substrate produced by a thick film formation method can be used as a substrate of the MCM, a thin film multi-layered wiring substrate using a polyimide having a lower dielectric constant than ceramics as an insulating layer and alternately laminating a layer of copper (Cu) wirings on this insulting layer has been developed and put into practical application in the field of large scale computers where higher speed of signals are required. (Refer, for example, to "Nikkei Micro-Device", December, 1989, pp 50-55.)
2. Description of the Related Art
The polyimide which has been developed and put into practical application as an insulating material for a wiring substrate has high heat resistance and a low dielectric constant ( =3.4).
However, this material is not devoid of the problem that it has high hygroscopicity due to its imide ring having high polarity, so that an apparent dielectric constant increases upon absorption of moisture and the insulating properties deteriorate.
Since a signal propagation speed is inversely proportional to .sup.1/2, the demand for materials having a lower dielectric constant than polyimides has increased.
A perfluorocarbon polymer (such as polytetrafluoroethylene (abbreviated to "PTFE")) is known as a material having low hygroscopicity, high heat resistance and an extremely low dielectric constant.
The perfluorocarbon resin as typified by PTFE is a material with a dielectric constant ( ) which is as low as about 2.0, and exhibits excellent heat resistance because the bond energy between carbon and fluorine is very high.
Furthermore, since this material has extremely low hygroscopicity and excellent chemical resistance, it has already been used as a substrate of a printed wiring board.
According to a thin film formation method it alternately laminates a wiring layer and an insulating layer individually, however, a rigid support substrate made of ceramics or a metal and the insulating layer, the insulating layer and a wiring metal, and the insulating layer and the insulating layer, must be adhered with one another. For this reason, PTFE has not been used as the insulating material for a multi-layered wiring substrate because adhesion of PTFE other than fusion has been difficult.
To apply PTFE to the thin film formation method, a method has been devised which renders PTFE porous by biaxial stretching, or the like, and then allows this porous film to be impregnated with a resin having high adhesion.
In other words, a dielectric constant and hygroscopicity can be reduced while retaining necessary adhesion and heat resistance by making PTFE composite with a resin.
The essential condition of the impregnation resin for PTFE according to this method is that porous PTFE can be impregnated with the resin and can be bonded by heat treatment, or the like.
As material satisfying this condition, a prepreg material "Gore-tex GTM-051 Series" (products of Japan Gore-tex Inc.) prepared by allowing porous PTFE to be impregnated with a bis-maleimide triazine resin (abbreviated to the "BT resin") as a low molecular weight thermosetting resin has been available commercially.
However, the BT resin has a considerably low heat resistance of not higher than 300.degree. C. and considerably high dielectric constant ( ) of more than 3.5. Therefore, even when this resin is made composite with porous PTFE, the dielectric constant of the resulting composite film cannot be reduced to 3.0 or less.
The perfluorocarbon polymers typified by PTFE are insulating materials having a low dielectric constant, but cannot as such be used for an inter-level insulating film of a multi-layered wiring substrate because their adhesion is extremely low.
On the other hand, a prepreg material prepared by allowing porous PTFE to be impregnated with the BT resin is commercially available for improving the adhesion of PTFE, but the heat resistance of the BT resin is up to about 300.degree. C. and the dielectric constant of the composite film is as great as about 3.0.
It is therefore a technical problem to obtain a thermosetting resin having excellent heat resistance but a low dielectric constant and capable of impregnation, to put an excellent multi-layered wiring substrate material into practical application by allowing porous PTFE to be impregnated with such a resin, and to form a multi-layered wiring substrate using an insulating material.
One of the important technical problems when forming a thin film multi-layered wiring is to form via holes of an inter-level insulating layer.
The thin film formation method employs the step of laminating a wiring layer individually in the same way as in the case of the formation of a semiconductor integrated circuit. For this reason, the use of reactive ion etching (RIE) has been examined for the formation of the via holes, too.
In the thin film circuit, however, the insulating layer is incomparably thicker than the semiconductor. To etch this thick insulating layer, a metal film having high dry etching resistance must be used as a resist layer. For this reason, a complicated process comprising the steps of: formation of the metal film.fwdarw.coating of a photoresist.fwdarw.exposure.fwdarw.development of the photoresist.fwdarw.etching of the metal resist.fwdarw.etching of the insulating film.fwdarw.removal of the metal resist.
In a thin film circuit using the polyimide as the insulating layer, therefore, a method that applies photosensitivity to a polyimide precursor and forms a via hole by utilizing photosensitivity of the material has been examined in order to simplify the fabrication process.
Nonetheless, the via hole cannot be formed by utilizing photosensitivity in the material impregnated with the thermosetting resin having adhesion to porous PTFE, and for this reason, the use of RIE requiring complicated steps becomes necessary.
Accordingly, a method of forming the via holes by a simple process must be accomplished for porous PTFE impregnated with the thermosetting resin.
As already described, the formation of the via holes utilizing photosensitivity cannot be carried out for those materials that are prepared by allowing porous PTFE to be impregnated with the adherable resin.
It is therefore important to materialize a method of forming the via holes by a simple and practical method.